5. THE RADIO LOBES

5.1. The Bridge and Plumes

At low frequency Cygnus A displays a `radio bridge' connecting the high
surface brightness regions at the source extremities
(Fig. 10).
Whether the radio bridge forms a physically connected structure through
the center of the galaxy, or just appears to in projection, remains an
open question. The comparison of radio and line emission in the center
of the Cygnus A galaxy
(Jackson et al. 1994,
Carilli et al. 1989b)
suggests that the lobes may not have completely evacuated the
inner 10 kpc of the galaxy.

At the center of the bridge are seen `plumes' of emission extending to
the north and south.
Leahy and Williams (1984)
and Williams (1985)
present a detailed analysis of radio bridges and bridges distortions in
powerful radio galaxies, including Cygnus A. The plumes in Cygnus A are
most consistent with Williams' model A in which back-flow of radio
emitting fluid in the lobe is deflected away from the high pressure
central regions of the galaxy by the general pressure gradient in the
ICM.